This invention relates to sol-gel processes for preparing tubular silica glass articles and more particularly, to a sol-gel process for preparing tubular silica glass articles having improved dimensional precision.
High purity silica glass is widely used as crucibles, boards and core tubes of diffusing furnaces used in the manufacture of semiconductor devices. In addition, high purity silica glass is useful for manufacturing glass instruments such as test tubes for chemical use, cells for optical measurement and substrates for thin film transistors. The demand for high purity glass is expected to expand significantly as a result of the large number of applications to which such glass has been and continues to be put.
Optical fibers formed primarily of silica glass are used as optical transmission media in the field of optical communication which includes information transmittance on a large scale. Preforms for these optical fibers are manufactured using silica glass tubes either as a starting material or to adjust the outer diameter of the preform. The silica glass tubes themselves must be of high quality and dimensionally precise so that high quality optical preforms can be made. The cost of manufacturing optical fibers is high due to the high cost of manufacturing the optical fiber preforms.
Prior art methods for manufacturing commercially available tubular silica glass (not necessarily for use in optical communication systems) include the following:
1. Washing and etching natural quartz crystal; PA1 2. Forming silicon oxide from high purity silicon tetrachloride or silicon tetrahydride; and PA1 3. Etching natural silica sand.
All of these prior art methods have disadvantages and it is extremely difficult to obtain tubular silica glass that meets the quality and dimensional precision required for use in optical communications systems using these methods. Furthermore, all of these methods require treatment of the glass at extremely high temperature and consequently, the cost of producing the glass is high. In fact, the third method is totally unsuitable for making optical fibers because of the low purity of the final product.
Recent attempts have been made to develop a sol-gel process that is useful for preparing silica glass tubes suitable for use as optical fibers. In general, sol-gel processes require use of an easily refined metal alkoxide as a starting material which permits high purity silica glass tubes to be obtained. In addition, transparent silica glass can be manufactured at temperatures lower than the transition point of the glass when sol-gel methods are used and consequently, the manufacturing cost is low.
An exemplary sol-gel process is disclosed in Japanese Laid-Open Patent Application No. 56-169121 of Hitachi. According to the process disclosed, a starting material consisting of silicon alkoxide, water, alcohol and a suitable catalyst is placed in a cylindrical container having a removable center bar. The material is gelled in the cylindrical container and the center bar is extracted when the material begins to shrink. Then the gel is dried and sintered to obtain a silica glass tube. Although this process is useful, it is difficult to manufacture large silica glass products without cracking and silica glass tubes of sufficient length cannot be obtained.
Another sol-gel method for manufacturing silica glass tubes using fumed silica as a starting material has been developed by Rabinovich. In the Rabinovich method the fumed silica is gelled and sintered and the resulting material is pulverized. The pulverized material is dispersed in a hydrosol solution which is then placed into a cylindrical container. A center bar is inserted into the sol solution in the container and the sol is gelled. After gelling the center bar is extracted to obtain a tubular gel. The tubular gel is dried and sintered and a glass tube having dimensions up to 1.7 cm inner diameter, 2.3 cm outer diameter and 25 cm length is obtained.
The Rabinovich process offers advantages over Hitachi because breaks and cracks occur less frequently during the formation and sintering steps and larger glass tubes can be obtained. However, the fact that two dispersion steps are required means that more impurities are likely to be introduced and the optical quality of the product may not be uniform.
The inventors have developed a sol-gel process that is suitable for preparing large silica glass articles. The process includes the steps of hydrolyzing a metal alkoxide to obtain a sol, adding fumed silica to the sol to obtain a sol solution, pouring the sol solution into a cylindrical container, gelling the sol solution while rotating the container around a tubular axis to obtain a tubular wet gel, drying the wet gel to obtain a dry gel and sintering the dry gel to obtain a tubular glass article. This process is disclosed in co-pending parent application, the specification of which is incorporated herein by reference as if fully set forth. However, it is desirable to provide even more uniformly dimensionally precise articles than can be prepared by the process of the parent application.
Accordingly, it is desirable to provide a sol-gel process for preparing dimensionally precise tubular silica glass articles which overcomes the disadvantages inherent in the prior art methods.